There are ignition coils to be used in internal combustion engines. Such an ignition coil has a primary coil, a secondary coil and power-source connector terminals to be connected to a power source. Patent document 1 discloses a structure of an ignition coil, in which a low voltage side of a secondary coil is connected to a power-source connector terminal connected to a power source through a diode. In the structure of the ignition coil disclosed in Patent document 1, a lead wire is connected to the diode and pressed and inserted into a slit which is formed at an end part of the power-source connector terminal. There is another ignition coil having a structure in which the low voltage side of the secondary coil is connected to the power-source connector terminal through a conductor wire instead of using the diode.